(a) The Cornea, Its Diseases and Current Treatments:
The cornea is the clear cover of the eye and is also the main focusing lens in the eye. Disorders of the cornea, which adversely affect its shape or clarity, can cause loss of vision. Such disorders include Fuchs' endothelial dystrophy, pseudophakic bullous keratopathy, keratoconus, and herpes virus infection. When these conditions are severe the most common treatment is a full thickness corneal transplant which is also known as penetrating keratoplasty.
Penetrating keratoplasty is the removal of a full-thickness disk of diseased corneal tissue followed by the replacement of the diseased full-thickness disk of tissue by a full thickness disk of donated healthy corneal tissue. Currently, the diseased tissue is removed by the use of a non-automated or automated corneal trephine combined with manual excision using scalpels and or micro-surgical scissors. The disk of donated healthy corneal tissue is then secured to the recipient cornea by the means of sutures using micro-surgical techniques. Penetrating keratoplasty can provide dramatic improvements in vision in patients who have opacified or irregularly shaped corneas. Approximately, 40,000 corneal transplants are performed annually in the United States.
However, there are distinct disadvantages of penetrating keratoplasty. For example, penetrating keratoplasty has a long recovery time and typically takes between 6 to 12 months to achieve good vision. Moreover, because the donor corneal tissue is sutured manually, even in the hands of an experienced corneal surgeon, irregularities in the shape of the cornea frequently occur and can produce decreased vision because of induced astigmatism. The donated corneal tissue can also be rejected by the recipient's immune system with resulting loss of transparency of the donated cornea. Penetrating keratoplasty also has the potential for a devastating complication called expulsive suprachoroidal hemorrhage. In this complication, a spontaneous hemorrhage from the choroidal blood vessels behind the retina can occur during penetrating keratoplasty surgery after the diseased cornea has been removed and before the donor cornea has been sutured securely in place. Because the eye is open to atmospheric pressure in this situation, there is no normal intraocular pressure to stop the choroidal vessels from bleeding. The terrible result is that the retina, vitreous, and crystalline lens may be expulsed from the opening in the cornea resulting in blindness. This complication is estimated to occur approximately 1 in 500 cases with penetrating keratoplasty. Endophthalmitis (i.e. infection of the inside of the eye) is another serious complication that can occur and can also cause blindness if treatment is unsuccessful. Finally, after penetrating keratoplasty, the eye is very sensitive to injury, since the junction of the transplanted cornea and the recipient cornea can be easily disrupted with even mild trauma.
Because of the disadvantages of penetrating keratoplasty other methods of corneal surgery have recently been developed, as follows.
Lamellar keratoplasty is the general term for corneal surgeries that involve cutting within the layers (lamellae) of the cornea. Lamellar keratoplasty techniques allow removal and replacement of specific layers of the cornea. It is useful to be able to remove and transplant specific layers of the cornea because there are common corneal conditions that involve only certain layers of the cornea.
For example, a scar in the cornea from a herpes virus infection may affect only the superficial layers of the cornea. Removal and transplantation of the superficial layers of the cornea may be all that is necessary to restore sight to an eye that has a superficial scar and avoids many of the complications that can be associated with penetrating keratoplasty including endophthalmitis and expulsive suprachoroidal hemorrhage.
Another example would be Fuchs' endothelial dystrophy. The endothelium is the innermost layer of the cornea, which is responsible for pumping fluid out of the corneal tissues. This removal of fluid prevents the cornea from swelling and becoming opaque. In Fuchs' endothelial dystrophy, the endothelium is damaged and is unable to adequately pump fluid out of the cornea, which results in swelling and opacification of the cornea. Removal of the diseased inner layers of the cornea and transplantation with a layer of healthy tissue can restore clarity to the cornea and vision to the eye. By only exchanging the inner layers of tissue, the front surface of the cornea is essentially undisturbed. This decreases the likelihood of post-surgical astigmatism and may also result in less risk of rejection of the transplanted tissue.
A particular technique of lamellar keratoplasty is anterior lamellar keratoplasty. Anterior lamellar keratoplasty is a procedure where the superficial layers of the cornea are separated from the deeper layers with a hand held scalpel or an automated corneal surgical device called a microkeratome. Using this technique, a cap of the superficial layers of the cornea is removed and then replaced with a healthy cap from the superficial layers of the donor cornea.
Unfortunately, corneal tissue removal and replacement by the free hand method is extremely difficult to perform. Under the best of circumstances, it usually results in irregular astigmatism that is caused by irregularities in the thickness of the corneal tissue removed as well as in the thickness of the transplanted tissue. The irregular astigmatism typically limits the best spectacle corrected vision to no better than 20/40.
As stated above, automated anterior lamellar keratoplasty involves the excision of a cap of superficial corneal tissue by the use of a microkeratome. Similarly, the same apparatus can be used to prepare a cap of superficial donor corneal tissue for transplantation. The donor tissue is then sutured to the recipient cornea. The sutures are typically removed within the first few months to minimize astigmatism. Unfortunately, a problem that can occur with this technique is that the transplanted donor disk may be dislodged with relatively minor trauma, even after prolonged periods of time. This can occur because the cap of corneal tissue is only held in place by the relatively weak healing between the layers of donor and recipient tissue and there is no support against lateral or vertical pressure.
Another particular technique of lamellar keratoplasty is posterior lamellar keratoplasty. Posterior lamellar keratoplasty is a procedure where the deeper (i.e. rear) layers of the cornea are separated from the superficial layers with a hand held scalpel or an automated microkeratome. A disk of the deeper layers of the cornea is removed and then replaced with a healthy disk from the deeper layers of the donor cornea.
In the free hand posterior lamellar keratoplasty technique, a blade is manually used to create a pocket in the deep layers of the cornea. An internal manual trephine is then used to cut a disk of the deepest corneal layers. The disk of the deepest corneal layers is then excised with microsurgical scissors and or scalpels. A disk of the deepest corneal layers is then placed inside the manually created pocket to fill the space of the excised corneal tissue. The transplanted disk of tissue initially stays in place by the pumping mechanism of the corneal endothelial cells and then gradually heals into place permanently. One significant advantage of this technique is that post-operatively, the eye is much less susceptible to injury than in other methods of corneal transplantation. Moreover, because the transplantation occurs within a pocket of the corneal tissues, the transplant is well protected by the intact boundaries of the corneal pocket. Unfortunately, the main disadvantage of such free hand technique is that it is very difficult to manually create a pocket in the corneal tissues, wherein the pocket is of uniform depth. Rather, it is instead quite possible to either prematurely cut through the deepest layers of the cornea and thus enter the anterior chamber, or to accidentally cut too superficially and thus exit from the superficial cornea. Unfortunately, the inability to create a uniform pocket will necessitate the abandonment of posterior lamellar keratoplasty and will require conversion to traditional penetrating keratoplasty.
Using a motorized microkeratome for posterior lamellar keratoplasty involves the creation of a flap of corneal tissue with a motorized blade. This is followed by excision of a disk of the deepest layers of the cornea including the endothelium. The excised disk of corneal tissue (including the endothelium) is replaced by the same layers from a donor cornea. The donated corneal disk is then secured in place with sutures. The corneal flap of the recipient cornea is also secured with sutures for up to several months. A disadvantage of this technique is that, like penetrating keratoplasty, the inside of the eye is exposed to atmospheric pressure and therefore there is also a risk of suprachoroidal hemorrhage with this technique. Another disadvantage is that post-operatively the eye is still fairly vulnerable to injury. For example, even minor trauma could result in flap dislocation or rupture of the transplant-recipient junction.
Recently anterior lamellar keratoplasty and posterior lamellar keratoplasty have also been performed on an experimental basis where the incisions have been created with a laser. Two disadvantages of this technique are the high cost of lasers and potential difficulty for the laser to create incisions in corneas that are scarred or opacified. See U.S. Pat. No. 6,325,792 to Swinger et. al.
(b) Treatment of Ametropia:
Ametropia, the incorrect focusing of light rays onto the retina, is the most common cause of decreased vision in humans. Common examples of ametropia include myopia, hyperopia or hypermetropia, and astigmatism. Because the cornea is the primary focusing lens in the eye, modification of the shape of the cornea by surgery has the ability to cause dramatic improvements in vision in patients that have ametropia.
LASIK (laser assisted in situ keratomileusis) is a method of laser vision correction that can dramatically improve vision by changing the shape of the cornea to allow the proper focus of light rays onto the retina. In the LASIK technique, a motorized blade is used to cut away a thin flap of tissue from the front of the cornea. The flap of corneal tissue is then lifted to expose the interior surface of the cornea. This exposed interior surface is then reshaped by the application of laser light. The flap of corneal tissue is then repositioned over the reshaped interior portion of the cornea. The flap initially stays in position through the natural pumping mechanism of the corneal endothelial cells and then gradually heals into place permanently. In this procedure, there is considerable variability in the size and shape of the laser treatment. However, with current corneal surgical devices the size and shape of the flap that covers the laser treatment is unfortunately rather limited.
Another vision improvement technique is keratophakia. Keratophakia is the insertion of a lens within the cornea. Keratophakia can also modify the curvature of the cornea for the purpose of improving a patient's vision. In Keratophakia, a pocket is made within the corneal tissues usually by means of a hand held blade. U.S. Pat. Application 20010004702 to Peyman describes a non-motorized apparatus for creating such a pocket within the cornea. In the Peyman device, movement of the blade is created by manually twisting the blade. After the pocket is made within the corneal tissue, an organic or synthetic lens is implanted within the pocket to reshape the cornea in order to change the focus of light rays. The disadvantage of either a manual technique or a non-motorized technique is that the uniformity of the pocket is largely dependent on the surgeon's skill and experience and therefore there can be a high degree of variability.
Because of the apparent difficulties with the current corneal surgical devices there is still a continuing need for an improved apparatus and method to create a pocket, flap, or a cap of corneal tissue in a live or donor cornea, wherein the pocket, flap, or cap is of uniform depth and thickness.